Semiconductors are widely used in integrated circuits for electronic applications, including high-speed computers and wireless communications. Such integrated circuits typically use multiple transistors fabricated in single crystal silicon. Many integrated circuits now contain multiple levels of metallization for interconnections. A single semiconductor microchip may have thousands, and even millions of transistors. Logically, a single microchip may also have millions of lines interconnecting the transistors. As device geometries shrink and functional density increases, it becomes imperative to reduce the capacitance between the lines. Line-to-line capacitance can build up to a point where delay time and crosstalk hinders device performance. Reducing the capacitance within these multi-level metallization systems reduces the RC constant, crosstalk voltage, and power dissipation between the lines. Typically, thin films of silicon dioxide are used as dielectric layers and to reduce the capacitance between functional components of the device.
Such dielectric thin films serve many purposes, including preventing unwanted shorting of neighboring conductors or conducting levels, by acting as a rigid, insulating spacer; preventing corrosion or oxidation of metal conductors, by acting as a barrier to moisture and mobile ions; filling deep, narrow gaps between closely spaced conductors; and planarizing uneven circuit topography so that a level of conductors can then be reliably deposited on a film surface which is relatively flat. A significant limitation is that typically interlevel dielectric (ILD) and protective overcoat (PO) films must be formed at relatively low temperatures to avoid destruction of underlying conductors. Another very important consideration is that such dielectric films should have a low relative dielectric constant k, as compared to silicon dioxide (k=3.9), to lower power consumption, crosstalk, and signal delay for closely spaced conductors.
Recently, attempts have been made to use materials other than silicon dioxide. Notable materials include low-density materials, such as aerogels and silsesquioxanes. The dielectric constant of a porous dielectric, such as a silicon dioxide aerogel, can be as low as 1.2. This lower dielectric constant results in a reduction in the RC delay time. However, methods of making aerogels require a supercritical drying step. This step increases the cost and the complexity of semiconductor manufacturing.
Films deposited from hydrogen silsesquioxane (HSQ) resins have been found to possess many of the properties desirable for ILD and PO applications. For example, Haluska et al. (U.S. Pat. No. 4,756,977, Jul. 12, 1988) describe a film deposition technique comprising diluting in a solvent a hydrogen silsesquioxane resin, applying this as a coating to a substrate, evaporating the solvent and ceramifying the coating by heating the substrate in air. Others have found that by ceramifying such a coating in the presence of hydrogen gas (Ballance et al., U.S. Pat. No. 5,320,868, Jun. 14, 1994) or inert gas (European Patent Application 90311008.8), the dielectric constant of the final film may be lowered and/or stabilized as compared to ceramifying in air. Each of these patents discloses the use of silsesquioxane resin dissolved in a solvent. The resulting silsesquioxane solution is coated onto a substrate by a spin-on coating technique. Although these coatings form useful dielectric layers after curing, as device sizes progressively minimize, it is necessary to have available dielectric thin films having a lower dielectric constant than that provided by the simple hydrogen silsesquioxane films.
Films comprising fluorinated siloxane components have low dielectric constants and are appropriate for integrated circuit applications. The fluorinated films are generally formed by CVD of a silicon-containing component and a small molecule that provides a fluorine source. Generally, this process requires the use of fluorine gas, which is both toxic and corrosive. Other methods utilize compounds that are both a source of carbon and fluorine. For example, Lee et al., U.S. Pat. No. 5,660,895, Aug. 26, 1997, discloses a process for the low temperature plasma-enhanced CVD of silicon oxide films using disilane as a silicon precursor and carbon tetrafluoride as a fluorine source.
Although limited effort has been directed towards chemical vapor deposition of silsesquioxane dielectric coatings. See, Gentle, U.S. Pat. No. 5,279,661, Jan. 18, 1994 disclosing CVD of hydrogen silsesquioxane coatings on a substrate, efforts to construct thin films from fluorinated silsesquioxanes have not been reported.
An array of low k thin films of different composition and precursors for these films which can be deposited onto a substrate using CVD would represent a significant advance in the art and would open avenues for continued device miniaturization. Quite surprisingly, the present invention provides such films and precursors.